9,10-dehydro-12,13-desoxyepothilone derivatives of the formula IV:
wherein R3 is methyl or trifluoromethyl, inhibit the growth of tumor cells and are therefore promising candidates for novel anticancer agents (see Danishefsky et al., J. Am. Chem. Soc. 2003, 125, 2899–2901; and International Patent Application Publication No. WO 2004/018478 A2).
The same authors (Danishefsky et al.) report a process for the preparation of epothilone derivatives which is described in scheme 1 below.

According to this process an olefin-precursor of formula 1 is converted in the presence of a Grubbs II catalyst of formula 2 to the respective epothilone derivative of formula 3 in a yield of 78%.
In the attempt to find an alternative synthesis which is feasible on a technical scale the objective of the present invention was to further improve the selectivity and the yield of the cyclization process.
It was found that with the process of the present invention, as outlined below, this improvement could surprisingly be achieved.